We have received many
inquiries about the feasibility of ANC in reducing
domestic and commercial noise problems. The conventional wisdom
seems to be that most noise situations are amenable to an ANC solution.
This is
false, just as is the notion that the computer is the solution to all
calculation and bookkeeping problems, or that computers reduce the
consumption of paper. Consider that most general noise problems are 3-dimensional in nature, typically a noisy area where one or more persons work or relax. Explanations are needed:
ANC uses the principle of destructive interference, where the noise
sound
wave is sensed with a microphone. This signal is amplified and fed to a
loudspeaker which emits sound of the same type but of opposite phase
with respect to the
original sound wave. Such cancellation can occur at only one point in
space. In the case of one-dimensional
propagation down a duct, the sound wave for all intents is "stopped" at
that point since the sound from the loudspeaker proceeds
one - dimensionally down the duct, matching the original noise waves
precisely
along the way. (upstream toward the sound source, the sound wave is
doubled, but that is of no concern. In the case of two or three dimensional propagation into space as into a room, The spreading of the loudspeaker sound does not match the pattern of the divergence of the noise field, having been produced a longer distance away. for widespread cancellation to occur, it requires that the loudspeaker be precisely placed at the center of the source of noise, usually an impossible task.
In more detail, one dimensional noise propagation allows ANC to occur
only
if the widest dimension of the noise channel (e.g. inside an air duct)
is
less than 1/4 wavelength. Therefore ANC in a duct one foot (0.3m) wide
will
not work above 275 Hz. Fortunately, such duct noise reduction is a
useful capability since noise
above 275 Hz is more easily absorbed by duct linings while noise below
275 Hz is
not, requiring some other means. ANC satisfies this need. The DIGISONIX
company (Wisconsin) has supplied such duct sound attenuation devices in
past years. A case of 3-D propagation, but with the cancellation very near to the reception point, is the ANC headset popular for pilots and other noise-field situations where it is acceptable to wear an earmuff type hearing protector. The fractional wavelength proximity requirement is easy to precisely achieve up to a frequency of about 500 Hz. Noise cancellation is truly outstanding at low frequencies down to 20 Hz or less (inside the ear cup only - at your ear), depending on the quality of ear cup seal around your ear. SENNHEISER, DAVID CLARK, BOSE, HEADSETS (a kit), TELEX, PELTOR and NOISEBUSTER are typical suppliers. The volume from which noise from common noise sources such as engines, transformers, exhaust fans, and loud outdoor music, emerges from is much more than a quarter wavelength across. (The "Emission" Problem.) Here, a single loudspeaker ANC transducer close to that noise source will not work because of that extent. A multiple loudspeaker ANC system can be built that allows noise cancellation in only one specific direction. Two common 3-dimensional noise problems are traffic noise arriving from a not-too-distant roadway, and noise already found inside a noisy room. (The "Immission" Problem.) Here, the 1/4 wavelength distance limit causes the effective volume where sound is canceled only in the immediate vicinity of the ANC loudspeaker. If the offending frequency is low (e.g. turboprop cabin noise at a passenger seat), noise cancellation of the propeller throb (about 100-200 Hz) is effective at a person's head position as long as that person does not move his or her head more than a few inches. Recent engineering efforts are extending this volume with multiple loudspeakers. In addition some attempts at driving the aircraft skin directly with piezoelectric transducers cemented to the aircraft skin are under way. Inquiries on the feasibility of ANC for noise (by the highway and at house windows) have been received recently. I have not given the inquirers any hope that this is feasible. If any one knows of progress in this matter, it should be reported in the alt.sci.physics.acoustics news group. (I know of only one experiment on this, conducted several years ago by the California DOT, no results were reported to my knowledge.) For more contemporary information, investigate the following links: http://www.signalsystemscorp.com/anc_faq.htm http://www.signalsystemscorp.com/anc_organizations.htm http://www.signalsystemscorp.com/ancFAQpart_ii.htm#typical Otherwise, perform a new web search on the subject: "Active Noise Control" Campanella
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